The art generally employs a structure known as a lead frame which spreads an array of leads, that are closely spaced around an aperture in which the integrated circuit chip is to be positioned, into an expanded and more easily connectable spacing of the array, at the periphery of the lead frame area. One illustration of a lead frame type structure is shown in U.S. Pat. No. 4,572,924.
There is an advantage in being able to join all contacts simultaneously in a single operation. To achieve this however all contacts should bond uniformly under the same joining conditions.
The technique of thermocompression bonding, wherein the heat in the bonding operation is accompanied by pressure sufficient for plastic deformation of the parts being bonded permits bonds to be made at lower temperatures.
There are a number of factors that operate to narrow the tolerances on the bonding conditions. Among them are very small registration variations allowed between the parts being bonded, lower heat tolerances arising from more diffusion-sensitive and oxidation-sensitive structures. This is particularly so for growth of aluminum oxide on the aluminum alloys generally used. Further, delamination of organic passivation layers within the device is aggravated by an elevated temperature.
The use of a slightly larger volume or bump of material at a particular location is employed to assist in registration and stress concentration in bonding. An illustration of the use of such bumps is shown in U.S. Pat. No. 4,188,438.
As the art has continued to progress, a variation of the lead frame technology has evolved in which a conductor pattern is placed on a tape type filament facilitating automated registration. This technology has become known as Tape Automated Bonding (TAB). The conductor lead ends are provided with bumps with the ends extending as beam leads cantilevered into the opening for the integrated circuit chip.
The TAB technology is described in the "Microelectronics Packaging Handbook" edited by R.R. Tummala and E.J. Rymaszewski, published by Van Nostrand, (1989) pages 409-431. In the process of TAB bonding, as the temperature is decreased, lower bond strengths are encountered. In IBM Technical Disclosure Bulletin, Vol. 30, No. 7, Dec. 1987, page 208, an improvement in thermocompression bonds is reported with less compression and shorter dwell time at the bonding temperature where at least one of the bonding surfaces is roughened.
Further, in conventional fusion metallurgy, it has been known that more reliable and uniform bonds can be achieved where the lead end to be bonded is first provided with a thin, fully-covering layer, and thereover a thicker, bonding layer is then provided. The covering layer is usually a non-oxidizing, preferably noble, metal.
One illustration of the technique is described in U.S. Pat. No. 3,873,428 in which gold is used as the contacting material, and the different layers are formed by plating, with a thicker gold layer being produced.
Another illustration of the two layer fusion contacting technique is described in French Patent 2,460,347 wherein the materials considered usable for the layers are gold, silver and nickel, the layers are formed by plating, but the difference in thickness between plated layers is achieved by varying current density and plating time.